It is well established by practice within the trenching service industry that it is desirable for the trench-cutting device to fracture the soil and excavate the spoils efficiently. Most trenchers include a tractor unit equipped with an elongated boom, which supports an endless chain power driven over sprockets at opposing ends of the boom. Cutting teeth are removably affixed to the chain.
There have been many attempts to develop teeth that accomplish both functions, but they have either been more efficient at fracturing or excavating. The variety of soil conditions affects the performance of each type of tooth.
Some of the early tooth designs were derived from mining breaker teeth. These teeth were hardened with a small area of contact and were manufactured to fracture the minerals. Many times, after the soil was fractured, another operation using different teeth was required to remove the fractured spoils from the trench.
Many different styles of hardened ripper or breaking teeth have been manufactured, as illustrated in U.S. Pat. Nos. 2,330,081, to. Phipps, and 3,954,301, to Stepp, they exemplify pointed and hardened teeth that are retained by being inserted in a housing that is affixed to the chain links.
The most common cutters used for excavation of spoils from a trench are cup tooth cutters; as illustrated in U.S. patents to Schmidt, 2,519,076 and Brown, 3,022,588. They are made from flat plate and are formed in a concave shape, generally so that the leading end is larger in radial diameter than the trailing end. This conical shape serves to carry the spoils up from the bottom as the cutter is carried along the carrier chain. Cup tooth cutters are effective when used in soft soils, but because of the relative lack of rigidity and temper, they abrade and erode quickly when used on hard mineral soils. The shape acts as a scoop or cup in which loose soils are carried out of the trench and then expelled when the cutter is inverted. Loose soils tend to fall free of the cutter as the cutter orientation is changed due to traveling up the circular path of the carrier chain, thus spilling back into the trench from which they were just removed. The concave cup area also tends to hold wet clay and sticky soils as it packs into the shape. Those soils tend to remain in the cutter despite inverting the cutter and must sometimes be pried or jarred loose. When cuttings pack into the cutter in this manner, the cutter cannot carry additional materials and the effectiveness of the cutters is greatly diminished. The result of the cutters carrying the additional weight of the packed soils and the resistance caused by the cutters being full as they are moved through the soil add additional loads to the power system driving the carrier chain.
U.S. Pat. No. 2,709,860, to Helton, describes a cutter that is comprised of a cup tooth body with an excavating breaker point attached to its exterior. The stated intent of the cutter was to fracture and excavate the spoils. The effectiveness of this design is questioned.
There have been other types and styles of cutting teeth manufactured for use in cutting trenches, such as U.S. Pat. Nos. 1,745,090, to Geithle, and 2,946,142, to Swanson, which are examples of teeth that are fabricated as right angles. These cutters are oriented and affixed to the carrier chain so that one leg is parallel to the chain links and the other leg is perpendicular to that leg and the chain link. The effect as the cutter is carried by the chain through the soil is to slice through it. The ability to carry spoils to the surface is minimal. These cutters are also ineffective as hard mineral fracturing teeth due to the blunt and relatively thin forward edges that contact the soil.
U.S. Pat. Nos. 3,614,838, to Wherry; 6,154,987, to Rumer et al; and 4,924,609, to Martin, all define angular cutter teeth that are affixed to the carrier chain links and are oriented so that the planer cutting area is parallel to the link. The cutting area of these teeth is bent at an angle that serves to widen the surface of the tooth as it is pulled into the soil. The bent angle is intended to cause the soil to fracture as well as to lift and excavate the spoils as it travels up through the trench. In Wherry the bend line between the mounting portion and the cutting portion is parallel to the chain movement rather than an acute angle. The projection 34 on Wherry creates substantial drag on the chain. The patent to Rumer et al has very little, if any, lifting of the spoils as also does the tooth of Martin.